Nanometer-sized metal materials, for example cobalt nanomaterials, may be used in electronics, high density data storage media (e.g., for recording media, and for memory devices), field sensors, catalysis, biotechnology and biomedical applications (e.g., cell sorting, diagnosis and drug delivery). The effectiveness of metal nanomaterials (“NMs”) used in such applications depends on the properties of the nanomaterials, for example, the degree of agglomeration, structure and shape, resistance to oxidation, and mechanical strength.
For example, magnetic properties of nanomaterials vary with particle size. Magnetic properties of small particles may be very sensitive to small thermal fluctuations. Thus when there is a wide size distribution, magnetic characteristics may be inconsistent throughout an agglomeration of nanoparticles. When the magnetic characteristics are varied, then such materials have limited application.
Most existing methods for generating metallic nanomaterials result in materials that are susceptible to rapid oxidation. As metallic nanomaterials oxidize, they tend to lose their magnetic properties.
Existing methods for generating nanomaterials include sputtering, chemical vapor deposition, reverse micelle synthesis, mechanical milling, solution phase metal salt reduction, and decomposition of neutral organometallic precursors. See, e.g., Murry et al. U.S. Pat. No. 6,262,129.
Numerous physical and chemical methods have been reported to provide controlled particle sizes and avoid agglomeration of cobalt nanoparticles, such as sputtering, (for example, see Kitakami, O.; Sato, H.; Shimada, Y.; Sato, F.; Tanaka, M. Phys. Rev. B, 1997, 21, 13849), chemical vapor deposition, (for example, see Billas, I. M. L.; Châtelain, A.; de Heer, W. A. J. Magn. Magn. Mater. 1997, 168, 64), reverse micelle synthesis (for example, see Petit, C.; Pilen, M. P. J. Magn. Magn. Mater. 1997, 166, 82), mechanical milling (for example, see Huang, J. Y.; Wu, Y. K.; Ye, H. Q. Acta Mater. 1996, 44, 1201), solution phase metal salt reduction (for example, see Murray, C. B.; Sun, S.; Gaschler, W.; Doyle, H.; Betley, T. A.; Kagan, C. R.; IBM J. Res. Dev, 2001, 45, 47), decomposition of neutral organometallic precursors (for example, see Masala, O.; Seshadri, R. Annu. Rev. Mater. Res., 2004, 34, 41), and high temperature reduction of salts such as CoCI2, CoI2, (for example, see Pelecky, D. L. L.; Bonder, M.; Martin, T.; Kirkpatrick E. M.; Liu, Y.; Zhang, X. Q.; Kim, S. H.; Rieke, R. D. Chem. Mater. 1998, 10, 3732), Co(CH3COO)2, (for example, see Murray, C. B. et al., supra), and Co(acac)3, (for example, see Cha, S. I.; Chan, B. M.; Kim, K. T.; Hong, S. H. J. Mater. Res., 2005, 20, 2148), using lithium and sodium compounds in the presence of stabilizing agents. The thermal decomposition of dicobalt octacarbonyl (DCO) under inert atmospheric conditions in the presence of surfactants is known to produce cobalt NMs of controlled size, shape and crystal structure, (for example, see Murray, C. B. et al.). Nanomaterials made by these methods tend to oxidize readily in air.
The orientation of crystal surfaces depends on the manner in which the atoms assemble. Hexagonally close packed (“hcp”) crystals appear to be the more stable form of Co. Further, hcp cobalt nanoparticles tend to be better for high density media, while face centered cubic (“fcc”) cobalt nanoparticles tend to be magnetically soft materials with low anisotropy. Epsilon (“ε”) crystals are another more complex cubic structure.
Use of surfactants in producing NMs is known to influence the crystal structure of the resulting materials. For example, the decomposition of DCO in the presence of the surfactant trioctylphosphine oxide (“TOPO”) has been reported to produce ε-cobalt nanoparticles. However, in the absence of TOPO, fcc cobalt nanoparticles were obtained. For example, see Dinega, D. P.; Bawendi, M. G. Angew., Chem. Int. Ed., 1999, 38, 1788. The synthesis of ε-cobalt nanoparticles by the thermal decomposition of DCO has been reported using the surfactants oleic acid and triphenyl phosphine, (for example, see Yang, H. T.; Shen, C. M.; Su, Y. K.; Yang, T. Z.; Gao, H. J.; Wang, Y. G., Appl. Phys. Lett., 2003, 82, 4729), or a mixture of surfactants composed of oleic acid (OA), lauric acid and trioctyl phosphine (TOP), (for example, see Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Appl. Phys. Lett. 2001, 78, 2187). The synthesis of multiply twinned fcc cobalt nanoparticles was reported by thermal decomposition of DCO in the presence of OA and tributyl phosphine, (for example, see Wang, Z. L.; Dai, Z.; Sun, S. Adv. Mater., 2000, 12, 1944). The ε-cobalt and fcc-cobalt phases required annealing at 300-500° C. to convert into the hcp phase, (for example, see Sato, H.; Kitkami, O.; Sakurai, T.; Shimada, Y.; Otani, Y.; Fukamichi, K. J. Appl. Phys. 1997, 81, 1858). Alivisatos et al. have reported direct synthesis of hcp Co nanoparticles, eliminating the need for annealing at high temperatures. (For example, see Puntes, V. F. et al.) The Chaudret group synthesized hcp Co nanoparticles by thermolysis of [Co(η3-C8H12)(η4-C8H12)], (for example, see Dumestre, F.; Chaudret, B.; Amiens, C.; Fromen, M. C.; Casanove, M. J.; Renaud, P.; Zurcher, P. Angew. Chem. Int. Ed., 2002, 41, 4286). Nanomaterials made by these methods tend to oxidize readily in air, however.
Baranauskas, (U.S. Pat. App. 20050196454) has proposed encapsulating nanoparticles with organic coatings to prevent oxidation by a complex synthetic method.
Bonnemann et al. have proposed encapsulating Co nanoparticles with Fe or FeOx to prevent oxidation of the cobalt, (see H. Bonnemann, R. A. Brand, W. Brijoux, W. W. Hofstadt, M. Frerichs, V. Voigts, and V. Caps Applied Organometallic Chemistry, 2005, 19, 790-796).
Behrens et al. proposed passivating Co-NM surfaces using “smooth oxidation” of the Co atom to prevent further oxidation of the particles (see Silke Behrens, Helmut Bonnemann, Nina Matoussevitch, Eckhard Dinjus, Harwig Modrow, Natalie Palina, Martin Frerichs, Volker Kempter, Wolfgang Maus-Friedrichs, André Heinemann, Martin Kammel, Albrecht Wiedenmann, Loredana Pop, Stefan Odenbach, Eckart Uhlmann, Nayim Bayat, Jürgen Hesselbach, and Jan Magnus Guldbakke, Z. Phys. Chem., 2006, 220, 3-40).
There is an unfilled need for simple method of making metallic nanomaterials that show air-stability.